A dense wavelength division multiplexed (DWDM) optical network, as with any information network, requires switches to perform routing of signals. DWDM networks pass several information channels along the same optical waveguide (e.g., optical fiber). Each channel corresponds to a different wavelength of light with the wavelengths typically separated by less than a nanometer. Consequently, DWDM networks require switches that are wavelength selective with very high resolution.
Regarding terminology, a 1×M optical switch comprises one input fiber (or port) and M output fibers (ports) to which the input can be selectively routed. A 1×M wavelength-selective switch similarly has one input port and M output ports and the capability of directing each of a number of discrete wavelengths from the input to any of the M output ports. The techniques of the invention may also be used to support M input fibers (ports) and I output fiber (port). Further, the techniques of the invention may be used to construct a system with M input fibers (ports) and M output fibers (ports).
For wavelength-selective optical switching, two commonly employed switching elements are micro-electromechanical (MEM) mirrors and liquid crystals (LC). These technologies use free space optics: the optical signal is removed from the fiber waveguide, manipulated using unguided optical components and then reinserted into an output fiber waveguide. Waveguided approaches (e.g., planar light circuits or PLCs) have been proposed for such functions but to date their promise has not been realized because of technical problems remaining to be overcome.
MEM micromirrors are constructed using microlithographic techniques. The mirrors are deformed or reoriented using electrostatic forces. Because of their small size and method of fabrication, it is straightforward to produce the arrays of mirrors required for wavelength-selective switching. Also, because the mirrors can take on a range of orientations they are conceptually easy to implement for higher port count wavelength-selective switches. It is the flexibility of the beam steering mechanism that makes MEM devices so promising and at the same time creates significant challenges for control and long term stability. MEM devices rely on steering a reflected beam; controlling the angle of reflection is paramount. Small deviations (<0.1 degree) in signal deflection can dramatically increase the coupling losses to an output port. Fabrication of the MEMs arrays requires an expensive processing facility, which makes them a costly solution for low volume applications.
Liquid crystal (LC) technology has a relatively long history in the prior art for optical switching applications. Liquid crystals are fluids that derive their anisotropic physical properties from the long range orientational order of their constituent molecules. Liquid crystals exhibit birefringence and the optic axis of a LC fluid can be reoriented by an electric field. This switchable birefringence is the mechanism underlying all applications of liquid crystals to optical switching and attenuation.
Two mechanisms have been proposed in the prior art for optical switching using liquid crystals: polarization modulation and total internal reflection (TIR). This refers to signal redirection to one of at least two channels (1×M switch; M>1). On/off liquid crystal optical switches can also be constructed on the principle of switchable scattering.
TIR liquid crystal switches rely on the difference in refractive index between the liquid crystal and the confining medium (e.g., glass). By proper choice of materials and angle of incidence of the light at the liquid crystal interface, it is possible to totally internally reflect the light when no field is applied to the liquid crystal. The effective index of the liquid crystal may be changed by reorienting the optic axis of the liquid crystal so that the total internal reflection criterion is no longer met; light then passes through the liquid crystal rather than reflecting from the interface. As with other types of reflective devices, such as MEM devices, controlling the reflection angle is critical. Also, since unwanted surface reflections are always present to some degree, crosstalk can be a significant problem.
Polarization modulation is the most common mechanism used in liquid crystal devices for optical switching. Switching is achieved between two orthogonal polarization states: for example, two orthogonal linear polarizations or left and right circular polarization. By way of illustration, a simple liquid crystal polarization modulator is shown in FIG. 1. The structure of the device is shown in cross-section FIG. 1a. A layer of nematic liquid crystal 1 is sandwiched between two transparent substrates 2 and 3. Transparent conducting electrodes 4 and 5 are coated on the inside surfaces of the substrates. The electrodes are connected to a voltage source 6 through an electrical switch 7. Directly adjacent to the liquid crystal surfaces are two alignment layers 8 and 9 (e.g., rubbed polyimide) that provide the surface anchoring required to orient the liquid crystal. The alignment is such that the optic axis of the liquid crystal is substantially the same through the liquid crystal and lies in the plane of the liquid crystal layer when the switch 7 is open. FIG. 1b depicts schematically the liquid crystal configuration in this case. The optic axis in the liquid crystal 10 is substantially the same everywhere throughout the liquid crystal layer. FIG. 1c shows the variation in optic axis orientation 12 as a result of molecular reorientation that occurs when the switch 7 is closed. The liquid crystal cell as described is known in the field as an electrically controlled birefringence device (or ECB). Such a liquid crystal polarization modulator was described in U.S. Pat. No. 5,276,747 as part of an optical switch/variable optical attenuator (VOA) for fiber optic communications applications.
To act as a switch, the modulator must produce two orthogonal polarizations at the exit of the modulator that can then be differentiated with additional optical components. This polarization conversion scheme provides the foundation for a number of electro-optic devices. If a linear polarizer is placed at the exit to the modulator, a simple on/off switch is obtained. If a polarizing beam splitter is placed at the exit, a 1×2 switch can be realized.
As an example, we consider a switchable half wave retardation plate. For this case, the liquid crystal layer thickness, d, and birefringence, Δn, are chosen so that
                                          Δ            ⁢                                                  ⁢            nd                    λ                =                  1          2                                    (        1        )            where λ is the wavelength of the incident light. In this situation, with reference to FIG. 1b, if linearly polarized light with wave vector 13 is incident normal to the liquid crystal layer with its polarization 14 making an angle 15 of 45 degrees with the plane of the optic axis 10 of the liquid crystal, the light will exit the liquid crystal linearly polarized with its polarization direction 16 rotated by 90 degrees from the incident polarization 14.
Referring now to FIG. 1c, the optic axis in the liquid crystal is reoriented by a sufficiently high field. If the local optic axis in the liquid crystal makes an angle Θ with the wave vector k of the light, the effective birefringence at that point is
                                          Δ            ⁢                                                  ⁢                          n              eff                                =                                                                      n                  e                                ⁢                                  n                  o                                                                                                                        n                      o                      2                                        ⁢                                          cos                      2                                        ⁢                    Θ                                    +                                                            n                      e                      2                                        ⁢                                          sin                      2                                        ⁢                    Θ                                                                        -                          n              o                                      ,                            (        2        )            where no and ne are the ordinary and extraordinary indices of the liquid crystal, respectively. The optic axis in the central region of the liquid crystal layer is nearly along the propagation direction 13. In this case, according to Eq. 2, both the extraordinary 17 and ordinary components 18 of the polarization see nearly the same index of refraction. Ideally, if everywhere in the liquid crystal layer the optic axis were parallel to the direction of propagation, the medium would appear isotropic and the polarization of the exiting layer 19 would be the same as the incident light 14.
Besides the ECB device described above, a number of other liquid crystal devices can operate as polarization switches: 90° degree twisted nematic, 270° twisted nematic, and ferroelectric LC are three common examples.
While it is easy to conceptualize a 1×2 LC-based switch, unlike the MEM device technology, generalization to 1×M with M>2 is problematic because of the discrete 2-state nature of the polarization switching. Cascading schemes have been proposed for broadband signal routing; however, these approaches are not amenable to wavelength-selective switching in a DWDM network.
The liquid crystal device of FIG. 1 is appropriate for broadband signal routing. However, for wavelength-selective switching, a liquid crystal device with more than one independently switchable element (pixel) is useful. A 1×N linear array of N separately addressable pixels is illustrated in FIG. 2. The structural cross-section of this LC array is identical to that of FIG. 1a. The liquid crystal is confined between two glass substrates 202 and 204 by means of a seal 206. Substrate 202 has one continuous electrode 208 which serves as the common electrode for the pixels. Substrate 204 has an array of N photolithographically patterned electrodes 210. Each pixel 212 is defined by the region of overlap between the common electrode and one of the patterned electrodes. By applying an independent voltage to each patterned electrode, the electro-optic response of each pixel in the array can be separately controlled.